7.3 Corrosion, Wear, Creep, Hydrogen Embrittlement & Fracture Mechanics
Key Takeaways
- Galvanic corrosion is accelerated by an unfavorable area ratio: a small anode paired with a large cathode results in an intensely concentrated corrosion current density at the anode ($i_a = i_c \frac{A_c}{A_a}$).
- Sensitization in austenitic stainless steels (304, 316) occurs when heating in the range of $450-850^\circ\text{C}$ causes chromium carbide ($Cr_{23}C_6$) precipitation at grain boundaries, depleting chromium below the $12\%$ passivity threshold.
- High-temperature creep becomes significant at homologous temperatures $T/T_m > 0.4$, and steady-state creep life is commonly extrapolated using the Larson-Miller Parameter: $LMP = T(C + \log_{10} t_r)$.
- Body-Centered Cubic (BCC) metals exhibit a distinct Ductile-to-Brittle Transition Temperature (DBTT) on Charpy V-notch impact tests, whereas Face-Centered Cubic (FCC) metals remain ductile even at cryogenic temperatures.
- Linear Elastic Fracture Mechanics (LEFM) dictates that catastrophic brittle fracture occurs when the stress intensity factor reaches the plane strain fracture toughness: $K_I = Y \sigma \sqrt{\pi a} \ge K_{Ic}$.
Last updated: August 2026
Corrosion, Wear, Creep, Hydrogen Embrittlement & Fracture Mechanics
Mechanical components frequently fail in service not from static mechanical overload, but from time-dependent degradation mechanisms including electrochemical corrosion, tribological wear, high-temperature creep, and brittle fracture. The NCEES PE Mechanical exam tests failure analysis principles, corrosion mitigation methods, creep life prediction models, and fracture mechanics criteria.
1. Fundamentals of Electrochemical Corrosion
Corrosion is the destructive electrochemical attack of a metal through chemical reaction with its surrounding environment. For wet (aqueous) electrochemical corrosion to proceed, all four components of a corrosion cell must be present:
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| THE FOUR-ELEMENT ELECTROCHEMICAL CELL |
| |
| Electrons flow through METALLIC PATH (e-) |
| +-------------------------------------------------+ |
| | | |
| v | |
| +-----------+ +-----------+ |
| | CATHODE | | ANODE | |
| | Reduction | | Oxidation | |
| | (Gains e-)| | (Loses e-)| |
| +-----+-----+ +-----+-----+ |
| | | |
| | Ions flow through ELECTROLYTE | |
| +<================================================+ |
| (e.g., Water + Dissolved O2 / Salts) |
+---------------------------------------------------------------------------------------------------------+
- Anode (Oxidation): The metal dissolves and loses electrons: $M \to M^{n+} + n e^-$ (Corrosion occurs here!).
- Cathode (Reduction): Consumes electrons. Common cathodic reactions in aerated neutral water: $O_2 + 2H_2O + 4e^- \to 4OH^-$; in acidic solutions: $2H^+ + 2e^- \to H_2 \uparrow$.
- Electrolyte: An ionic conductor (moisture, soil, seawater) allowing ion transport.
- Metallic Path: Direct electrical connection allowing electron transfer between anode and cathode.
2. Common Forms of Corrosion & Diagnostic Criteria
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| TAXONOMY OF CORROSION MECHANISMS |
| |
| UNIFORM ATTACK GALVANIC CORROSION CREVICE CORROSION PITTING CORROSION |
| - Even metal loss - Dissimilar metals - Shielded gaps/gaskets - Localized passive film|
| - Predictable mpy rate - Small anode / large cathode- O2 depletion cell breakdown (PREN metric)|
| |
| INTERGRANULAR (IGC) STRESS CORROSION (SCC) EROSION-CORROSION HYDROGEN EMBRITTLEMENT |
| - Cr23C6 at grain bndry - Tensile stress + corrosive - Fluid turbulence + - H+ absorption into |
| - HAZ sensitization environment wear at pipe elbows high-strength steels |
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Detailed Corrosion Forms
| Corrosion Mechanism | Environmental / Material Triggers | Diagnostic Features & Mitigation |
|---|---|---|
| Uniform (General) | Atmospheric exposure, general acidic baths | Even reduction in wall thickness over entire surface. Quantified in mils per year ($\text{mpy} = \frac{534 W}{D A t}$). Managed via corrosion allowance thickness. |
| Galvanic | Dissimilar metals in direct electrical contact within an electrolyte | Driven by galvanic potential difference. Area Rule: A small anode connected to a large cathode causes extreme localized galvanic penetration at the anode. Mitigate with dielectric isolation bushings or sacrificial anodes. |
| Crevice Corrosion | Micro-gaps under bolt heads, lap joints, gaskets | Differential aeration cell: oxygen is depleted in the crevice, concentrating $Cl^-$ ions and dropping pH ($Fe^{2+} + 2Cl^- + 2H_2O \to Fe(OH)_2 + 2HCl$). Mitigate by seal welding or non-wicking gaskets. |
| Pitting Corrosion | Halide ions ($Cl^-$) attacking passive oxide films ($Al_2O_3$, $Cr_2O_3$) on stainless steel and aluminum | Extremely rapid, localized autocatalytic downward tunneling. Characterized by Pitting Resistance Equivalent Number: $\text{PREN} = %Cr + 3.3%Mo + 16%N$. High PREN alloys (e.g., 316 SS or Super Duplex) resist pitting. |
| Intergranular Corrosion (Sensitization) | Heating austenitic stainless steels (304, 316) to $450-850^\circ\text{C}$ (welding HAZ) | Chromium carbide ($Cr_{23}C_6$) precipitates along grain boundaries, dropping adjacent grain boundary Chromium below $12%$ (loss of passivity). Grains fall out like sand. Mitigate using low-carbon grades (304L/316L, $C < 0.03%$) or stabilized grades (321 with Titanium, 347 with Niobium). |
| Stress Corrosion Cracking (SCC) | Simultaneous sustained static tensile stress + specific corrosive environment | Sub-critical crack propagation at stresses far below $S_y$. Classic pairs: Austenitic SS + hot chlorides ($>60^\circ\text{C}$); Brass + ammonia ("season cracking"); High-strength carbon steel + caustic/sulfides. Mitigate by stress relieving (PWHT) or material substitution. |
| Hydrogen Embrittlement | Ingress of atomic hydrogen ($H$) into high-strength steels ($S_y > 100 \text{ ksi}$) | Hydrogen atoms coalesce at high triaxial tensile stress concentrations, causing sudden catastrophic brittle intergranular cracking. Mitigate by post-plating baking at $190-220^\circ\text{C}$ for 4-24 hours. |
3. Corrosion Prevention & Protection Methods
- Material Selection: Select corrosion-resistant alloys (CRAs) or non-metallics (FRP, PVC, PTFE).
- Barrier Coatings: Epoxies, polyurethanes, paint systems, and thermal spray coatings.
- Galvanizing (Hot-Dip Zinc): Provides dual protection: acts as a physical barrier and serves as a sacrificial anode if the coating is scratched.
- Cathodic Protection (CP): Makes the protected structure the cathode of the electrochemical cell:
- Sacrificial Anode (Galvanic): Connects a more active metal (Zinc, Magnesium, Aluminum) to the steel structure. The sacrificial anode corrodes preferentially.
- Impressed Current Cathodic Protection (ICCP): Connects an external DC power supply with inert anodes (mixed metal oxide / platinum) to drive protective electrons into the structure.
4. Mechanical Wear Mechanisms
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| PRIMARY WEAR MECHANISMS |
| |
| 1. ADHESIVE WEAR Microscopic asperity welding under load and subsequent shearing (galling). |
| 2. ABRASIVE WEAR Hard particles or asperities plow grooves into softer surface. Archard's |
| law: Volume wear V = K * (W * L) / H. (Two-body vs. three-body). |
| 3. EROSIVE WEAR Solid particles or liquid droplets impacting surface at high velocity. |
| 4. FRETTING WEAR Micro-oscillatory motion (<100 um) under contact load (fretting corrosion). |
| 5. SURFACE FATIGUE Cyclic Hertzian contact stresses causing subsurface microcracks and spalling.|
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5. High-Temperature Creep & Larson-Miller Parameter
Creep is the progressive, time-dependent plastic deformation of a material subjected to constant mechanical stress at elevated homologous temperatures ($T / T_m > 0.4$, where $T_m$ is absolute melting point in Kelvin or Rankine).
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| TYPICAL CONSTANT-LOAD CREEP CURVE |
| |
| Strain (epsilon) |
| ^ Tertiary Stage |
| | (Accelerating / Rupture) |
| | /------X (Rupture point: t_r) |
| | Secondary Stage / |
| | (Steady-State Creep: deps/dt)/ |
| | +--------------------------------+ |
| | / |
| | Primary / |
| | Stage / |
| | / |
| | Instantaneous |
| | Elastic Strain |
| +--+---------------------------------------------------------------------> Time (t) |
+---------------------------------------------------------------------------------------------------------+
The Three Creep Stages
- Primary (Transient) Creep: Creep rate $d\epsilon/dt$ decreases continuously with time due to work hardening.
- Secondary (Steady-State) Creep: Constant minimum creep rate $\dot{\epsilon}_{ss}$ where strain hardening balances thermal recovery. Primary design parameter for long-life boilers and turbine rotors.
- Tertiary Creep: Rapid acceleration of creep rate due to internal void formation, grain boundary cavitation, and localized necking, leading to rupture ($t_r$).
The Larson-Miller Parameter ($LMP$)
The Larson-Miller Parameter allows trading temperature and rupture time along a unified master curve:
Where:
- $T = \text{absolute temperature in Kelvin } (\text{K}) \text{ or Rankine } (^\circ\text{R})$
- $t_r = \text{time to rupture in hours}$
- $C = \text{Larson-Miller constant (typically } C \approx 20 \text{ for steels and nickel superalloys)}$
[!IMPORTANT] When evaluating $LMP$ in Rankine: $T(^\circ\text{R}) = T(^\circ\text{F}) + 459.67$. When in Kelvin: $T(\text{K}) = T(^\circ\text{C}) + 273.15$. Always verify whether the master curve is calibrated with $T$ in Kelvin or Rankine.
6. Impact Toughness & Fracture Mechanics (LEFM)
Charpy V-Notch (CVN) & Ductile-to-Brittle Transition Temperature (DBTT)
- BCC Metals (Ferritic Steels): Exhibit a steep transition from ductile shear failure (high absorbed energy, fibrous appearance) to brittle cleavage fracture (low absorbed energy, reflective faceted appearance) across a narrow temperature band (DBTT).
- FCC Metals (Austenitic Stainless Steels, Aluminum, Copper): Do not possess a DBTT; they maintain high impact energy absorption even at liquid nitrogen (cryogenic) temperatures.
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| CHARPY IMPACT ENERGY VS. TEMPERATURE (DBTT) |
| |
| Impact Energy (Joules / ft-lb) |
| ^ |
| | FCC Metals (Austenitic SS, Al) ------------------------------------ (No DBTT) |
| | Upper Shelf (Ductile) |
| | +------------------------ |
| | / |
| | BCC Carbon Steels / |
| | / <- Transition Region (DBTT) |
| | / |
| | Lower Shelf (Brittle) / |
| | +-----------------------------+ |
| +-----------+-----------------------------+-----------------------------> Temperature |
| Cryogenic DBTT Room/High |
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Linear Elastic Fracture Mechanics (LEFM)
For a component containing an internal crack of length $2a$ (or edge crack of depth $a$) under nominal tensile stress $\sigma$:
Where:
- $K_I = \text{Mode I stress intensity factor } [\text{ksi}\sqrt{\text{in}} \text{ or } \text{MPa}\sqrt{\text{m}}]$
- $Y = \text{dimensionless geometric correction factor (typically } Y = 1.0 \text{ for wide center crack, } 1.12 \text{ for edge crack)}$
- $\sigma = \text{nominal applied gross tensile stress}$
- $a = \text{crack length (half-length for internal crack, total depth for edge crack)}$
Fracture Criterion: Catastrophic rapid brittle fracture occurs when the stress intensity factor reaches the material's critical plane strain fracture toughness ($K_{Ic}$):
7. Step-by-Step Worked Problem: Creep Rupture Life Extrapolation
Problem Statement
A high-pressure superheater steam tube made of a Cr-Mo alloy steel is operating at $1050^\circ\text{F}$ under an internal pressure inducing a hoop stress of $15 \text{ ksi}$. Laboratory creep test data shows that at this exact stress level ($15 \text{ ksi}$), the alloy exhibits a Larson-Miller Parameter of:
Calculate:
- The predicted creep rupture life ($t_r$) of the tube in hours and years at the design operating temperature of $1050^\circ\text{F}$.
- The predicted rupture life if an operational excursion causes the tube temperature to increase by $50^\circ\text{F}$ to $1100^\circ\text{F}$.
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| STEP-BY-STEP SOLUTION PROCEDURE |
| |
| STEP 1: Calculate Rupture Life at Design Temperature (1050 deg F) |
| Convert temperature to absolute Rankine: |
| T_1 = 1050 + 459.67 = 1509.67 deg R |
| Larson-Miller equation: LMP = T_1 * (20 + log10(t_r1)) |
| 36,000 = 1509.67 * (20 + log10(t_r1)) |
| 20 + log10(t_r1) = 36,000 / 1509.67 = 23.846 |
| log10(t_r1) = 23.846 - 20 = 3.846 |
| t_r1 = 10^(3.846) = 7,015 hours (approximately 0.80 years of continuous operation) |
| |
| STEP 2: Calculate Rupture Life at Excursion Temperature (1100 deg F) |
| Convert excursion temperature to Rankine: |
| T_2 = 1100 + 459.67 = 1559.67 deg R |
| Larson-Miller equation (stress remains 15 ksi, so LMP is unchanged at 36,000): |
| 36,000 = 1559.67 * (20 + log10(t_r2)) |
| 20 + log10(t_r2) = 36,000 / 1559.67 = 23.082 |
| log10(t_r2) = 23.082 - 20 = 3.082 |
| t_r2 = 10^(3.082) = 1,208 hours (approximately 0.14 years) |
| |
| ANALYSIS: A mere 50 deg F temperature increase reduces the creep rupture life from 7,015 hours down |
| to 1,208 hours—an 82.8% reduction in equipment service life! |
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8. Common Exam Traps & PE Pro-Tips
- Trap 1 — Absolute Temperature in Larson-Miller Parameter: Forgetting to convert $^\circ\text{F}$ to $^\circ\text{R}$ ($+459.67$) or $^\circ\text{C}$ to $\text{K}$ ($+273.15$). The formula is completely invalid with gauge temperatures.
- Trap 2 — Crack Length Definition in Fracture Mechanics: For a center crack through a plate, the total flaw length is $2a$, so you must divide by 2 to find $a$. For an edge crack, the measured flaw depth is $a$.
- Trap 3 — Unfavorable Galvanic Area Ratio: Anodic vs. cathodic areas govern galvanic severity. Fastening a large aluminum panel with small steel rivets is safe (small cathode, large anode $\implies$ negligible steel corrosion rate), whereas fastening large steel plates with small aluminum rivets causes catastrophic rapid rivet failure.
Test Your Knowledge
A piping engineer is coupling an aluminum pipe spool directly to a large carbon steel heat exchanger shell immersed in seawater. Which of the following conditions represents the most severe galvanic corrosion hazard?
A
B
C
D
Test Your Knowledge
Which metallurgical mechanism causes sensitization in welded standard AISI 304 austenitic stainless steel piping?
A
B
C
D
Test Your Knowledge
A wide steel plate with an edge crack of depth a = 0.25 inches (Y = 1.12) has a plane strain fracture toughness K_Ic = 45 ksi*sqrt(in). What is the critical gross tensile stress that will cause catastrophic brittle fracture?
A
B
C
D
Test Your Knowledge
Which family of engineering alloys does NOT exhibit a ductile-to-brittle transition temperature (DBTT) and remains ductile with high Charpy impact energy absorption at cryogenic temperatures?
A
B
C
D